Lyme disease, which is caused by infection with Borrelia burgdorferi and related spirochetes, is transmitted by ticks, and is characterized by dematologic, neurologic, cardiovascular, and arthritic manifestations. B. burgdorferi is an invasive organism that causes disease through persistent infection and chronic stimulation of host inflammatory responses. Understanding how the organism can evade the host immune response and cause persistent infection is important for understanding its pathogenesis, and improving diagnosis, treatment, and prevention. In the previous period of the award, Dr. Norris and colleagues discovered that B. burgdorferi possesses an elaborate system of antigenic variation, called the VMP-like Sequence (vls) locus, because of its similarity to the Variable Major Protein system of relapsing fever Borrelia. A central cassette region of the vlsE gene has been identified which undergoes extensive segmental recombination with a series of vls silent cassettes located adjacent of vlsE on the linear plasmid lp28-1. VlsE protein is highly immunogenic, and immunization with it confers protection against infection with B. burgdorferi expressing the homologous protein, but is only partially protective against isogenic strains expressing variants of VlsE, generated by recombination in the vls locus. Additionally, a strong antibody response can be detected in animals and in humans infected with B. burgdorferi, indicating VlsE is immunogenic and expressed during infection. These findings indicate the discovery of a highly immunogenic protein whose ability for genetic and immunologic variation could provide the bacteria with a powerful mechanism for immune evasion. The investigator plans to build on these initial important findings to further characterize the vls system in related spirochetes that cause Lyme disease, B. garinii, B. afzellii, and other isolates of B. burgdorferi. The immune response to VlsE epitopes will be investigated, and the potential of a multi-valent vaccine based on variant epitopes of VlsE will be tested. The location of invariant and variant epitopes of VlsE will be determined by structural analysis, employing recombinant VlsE and making use of monoclonal antibodies to be generated against VlsE variants. The correlation of particular plasmids with infectivity will be further analyzed to help identify additional gene products important in persistence, invasion, and pathological development.